Current approaches for assessing molecular endpoints in certain diseases usually require tissue and blood sampling, surgery, and in the case of experimental animals, sacrifice at different time points. Despite improvements in noninvasive imaging, more sensitive and specific imaging agents and methods are urgently needed. Imaging techniques capable of visualizing specific molecular targets and/or entire pathways would significantly enhance our ability to diagnose and assess treatment efficacy of therapeutic interventions for many different disease states. Most current imaging techniques report primarily on anatomical or physiological information (e.g., magnetic resonance imaging (MRI), computed tomography (CT), and ultrasound). Newer modalities such as optical imaging and new molecular imaging probes have the potential to revolutionize the way disease is detected, treated, and monitored.
In particular, optical imaging offers several advantages that make it a powerful molecular imaging approach, both in the research and clinical settings. Specifically, optical imaging can be fast, safe, cost effective and highly sensitive. Scan times are on the order of seconds to minutes, there is no need for ionizing radiation, and the imaging systems can be relatively simple to use. In addition, optical probes can be designed as dynamic molecular imaging agents that may alter their reporting profiles in vivo to provide molecular and functional information in real time. In order to achieve maximum penetration and sensitivity in vivo, the choice for most optical imaging in biological systems is within the red and near-infrared (NIR) spectral region (600-900 nm), although other wavelengths in the visible region can also be used. In the NIR wavelength range, absorption by physiologically abundant absorbers such as hemoglobin or water, as well as tissue autofluorescence, is minimized.
Hypoxia, or hypoxiation, is a pathological condition in which the body as a whole (generalized hypoxia) or a region of the body (tissue hypoxia) is deprived of adequate oxygen supply. Variations in arterial oxygen concentrations can be part of the normal physiology, for example, during strenuous physical exercise. A mismatch between oxygen supply and its demand at the cellular level may result in a hypoxic condition.
Tumor hypoxia is the situation where tumor cells have been deprived of oxygen. As a tumor grows, it rapidly outgrows its blood supply, leaving portions of the tumor with regions where the oxygen concentration is significantly lower than in healthy tissues. It can also be a result of the high degree of cell proliferation undergone in tumor tissue, causing a higher cell density, and thus taxing the local oxygen supply.
The carbonic anhydrases (or carbonate dehydratases) form a family of enzymes that catalyze the rapid interconversion of carbon dioxide and water to bicarbonate and protons (or vice-versa), a reversible reaction that occurs rather slowly in the absence of a catalyst. One of the functions of the enzyme in animals is to interconvert carbon dioxide and bicarbonate to maintain acid-base balance in blood and other tissues, and to help transport carbon dioxide out of tissues.
Carbonic anhydrases (CAs) are a large family of zinc metalloenzymes that participate in a variety of biological processes, including respiration, calcification, acid-base balance, bone resorption, and the formation of aqueous humor, cerebrospinal fluid, saliva, and gastric acid. They show extensive diversity in tissue distribution and in their subcellular localization. CA IX is a transmembrane protein that has been shown to be significantly upregulated under hypoxic conditions and plays a critical role, along with the intracellular carbonic anhydrase II, in hypoxia associated extracellular acidifcation. It is expressed in all clear-cell renal cell carcinoma, but is not detected in normal kidney or most other normal tissues. It may be involved in cell proliferation and transformation. Importantly, tumor hypoxia and subsequent expression of CA IX are associated with poor prognosis and treatment outcomes in numerous cancer types. The ability to more accurately and efficiently detect and quantify carbonic anhydrase—associated hypoxia will aid in the understanding of biological phenomena such as cellular proliferation and cancer, as well as in the determination of the most appropriate treatment regimens.